How long before Google kills Google voice?
Using the reasoning they uesed to kill Reader, I can't imagine them keeping voice around much longer. I'm sure it has far less users and obviously has very little development resources allocated to it.
I don't expect it will be killed outright, but my guess is that some of its features will be folded into some other Google product (Hangouts? Google Chat?) and users will be told to migrate.
It's still a US-only service after many years. If it were lucrative, they probably would have moved it into other markets by now.
Can't you just keep your number when changing phone companies?
http://www.gwern.net/Google%20shutdowns#predictions
44% chance of surviving the next 5 years.